As for the transfer paper for xerography, non-coated papers such as wood free paper and the like have been prevailingly used. Even when conventional coated papers for graphic arts including art paper are used, however, high quality images cannot be obtained.
As a reason for it, it can be thought that prior to thermal fixation a satisfactory toner image can be obtained on art paper or another coated paper since such paper can come into uniform contact with a photoreceptor upon copying operation because of its very high surface smoothness, but it becomes difficult for the paper upon thermal fixation to retain the whole toner thereon because the surface layer of the paper is too compact, and thereby part of toner is scattered.
Further, ordinary coated papers for graphic arts are usually insufficient in gas permeability, because various kinds of adhesives are admixed in relatively large amounts with the intention of enhancing the surface gloss after printing and securing the surface strength upon printing.
When a coated paper as described above is used as transfer paper for xerography, it is known that the coated paper causes a so-called blister phenomenon, or a phenomenon such that the coated paper cannot completely let out the vapor generated therein by its moisture being quickly heated through the fixation with heating rollers, thereby causing interlayer separation in the coated layer part or the raw paper part. This blister phenomenon roughens images (Japanese Journal of Paper Technology, vol. 27, No. 4 (1984), pp. 31-36).
As preventive measures taken against the blister phenomenon, there are known, for instance, the method of controlling both surface roughness and air permeability to no more than 4,000 seconds (Tokko Hei 5-82939 and Tokko Hei 5-82940, wherein the term "Tokko" means "examined Japanese patent publication"); the method of providing a coated layer having a coverage rate of 2 to 5 g/m.sup.2 and a sufficient water vapor transmission rate, which is controlled within the range of 50 to 500 g/m.sup.2 .times.24 hours, on each side of a raw paper having a limited air permeability (Tokkai Hei 1-245265, wherein the term "Tokkai" means "unexamined published Japanese patent application"); and the method of providing a coated layer having surface roughness not more than 2.0 .mu.m and a surface electric resistance not less than 8.times.10.sup.8 .OMEGA. at 20.degree. C., 85% RH (Tokkai Sho 62-198877).
Most of those coated papers are, however, low in surface gloss before they undergo a printing operation. Although some of them are originally high in surface gloss, they have a defect such that their gloss balance is not good as a whole because of their low image gloss in the halftone area having a small quantity of toner, while in the solid area having a large quantity of toner they suffer from the blister phenomenon. Accordingly, the coated papers as described above are unsuitable to transfer paper for xerography when high image quality equal to that attainable with high-grade graphic arts paper and photographic printing paper, especially uniform and high image gloss, is required thereof irrespective of the quantity of toner.
On the other hand, a thermal transfer system comes into wide use at the present time. The system uses ink sheets colored yellow, magenta and cyan, or three primary colors, respectively. Each of the ink sheets has on a support an ink coating containing a heat-fusible compound and a coloring material as main components. Each ink sheet is brought into a face-to-face contact with a transfer paper, and heat is applied thereto with a thermal head to transfer the ink onto the transfer paper. Thus, in analogy with color printing, a full color recording is obtained on the transfer paper by the foregoing three primary colors' being variously overlapped. Therefore, the amount of ink transferred on the transfer paper in this system is two or three times as much as that in the system for monochromatic recording. The ink image formed on the transfer paper becomes more nonuniform as the first layer (yellow), the second layer (magenta) and the third layer (cyan) are superposed successively. That is, the ink image formed does not have satisfactory quality. For instance, a green-colored solid area is formed by superposition of cyan ink upon yellow ink. In this case, the solid area thus formed involves parts having colors other than green in viewing it microscopically unless both yellow ink and cyan ink are uniformly transferred. This phenomenon is called the running-over phenomenon of ink, which is a current serious problem.
If plain paper such as copying paper is used as transfer paper in the foregoing thermal transfer system also, the running-over problem of ink can be solved. However, using plain paper as the transfer paper has a counterbalancing disadvantage in marked deterioration of image quality resulting from much roughness of the paper surface. More specifically, transfer unevenness is caused by insufficient contact between the paper surface and the thermal head, and transferred colors are deficient in clearness and density due to too much ink permeation into the paper.
Accordingly, in general printers and copying machines of thermally fused ink transfer type, wood free paper having undergone a surface treatment for heightening the smoothness, such as a supercalendering treatment, has been used as transfer paper. In particular, images recorded on such a surface-treated paper are known to become clear when the paper has smoothness of not less than 100 seconds on the recording side. This is because the paper surface can be brought into closer contact with the ink donor sheet upon recording by virtue of its heightened smoothness on the recording side. Such paper has fairly good image reproducibility in a solid image area, as described above, while in a halftone image area the image reproducibility thereof is still insufficient.
Thus, a countermeasure that suggests itself is to use coated papers having higher smoothness. However, it is the present state of things that coated papers generally used for graphic arts which have high smoothness and high surface gloss are not used as the transfer paper for the thermally fused ink transfer process. This is because a transferred ink image on coated paper can have good quality in theory since the coated paper can be in uniform contact with an ink donor sheet upon recording because of its very high surface smoothness, but in fact the image reproduced on the coated paper does not have satisfactory quality as the ink is not uniformly transferred to the coated paper. This tendency is more pronounced when art paper or another coated paper for graphic arts, which has particularly high smoothness and surface gloss, is used as transfer paper.
As reasons for it, the following can be thought: Since such a coated paper for graphic arts is made highly smooth by the use of a surface-treatment apparatus, usually a supercalender, the surface part of the coated layer is poor in roughness and void. Thus, there are caused troubles such that fused ink cannot firmly hold on to the surface of the transfer paper (that is, it has poor anchorage), and the ink once transferred to the paper is retransferred to the ink donor sheet through the running-over phenomenon of ink.
Such being the case, various measures have been proposed. Examples thereof include the method of coating an aqueous coating material comprising a water-soluble adhesive and a pigment on a paper sheet to prepare a thermal ink-transfer recording material (Tokko Sho 59-16950), the method of using an oil absorbing pigment having oil absorption of not less than 30 ml/100 g (Tokkai Sho 57-182487), the method of adding fine particles of a vinyl polymer having a particle size of from 0.1 to 1.0 .mu.m and Tg of not lower than 80.degree. C. (Tokkai Sho 60-38192), and the method of using a nonionic water-soluble polymer having a low polymerization degree and a porous pigment having oil absorption of from 30 to 200 ml/100 g (JIS-K5101)as main components (Tokko Hei 5-19919 and Tokko Hei 5-78439).
However, all the products obtained adopting the above-cited methods are originally low in surface gloss, and have a so-called matte surface. Therefore, the gloss of the images printed thereon (i.e., image gloss) is increased with an increase in quantity of ink, although the density thereof is rather high. As a whole, the images printed thereon cannot have such high and uniform gloss as those formed in photographic printing paper have. That is, those products are inferior in image gloss balance, so they fail in providing images of high quality. It can be thought that this failure is attributable to the use of a surface-treatment apparatus, such as a supercalender, with the intention of improving the surface properties. More specifically, it can be considered that the use of a surface-treatment apparatus causes reduction of voids and roughness at the paper surface, and thereby the paper surface cannot have an oil absorbing power high enough to receive images.
As described above, it is the present condition that there are not yet known thermal transfer papers of the type which can provide, in full-color printing, images of high quality with respect to image characteristics including reproducibility, sharpness, gradation and so on, and image gloss as a whole, even in the halftone image area having a small quantity of ink.
As a result of our intensive studies for solving the above-described problems, it has been found out that when the printing utilizing xerography or a thermally fused ink transfer process is carried out the void structure of transfer paper, especially the void structure at the transfer layer surface, plays an important role in providing the printed images with high and uniform gloss as a whole irrespective of the quantity of toner or ink, thereby achieving the present invention.